Field of the Invention
The present application relates to a laminated diffraction optical element and a production method therefor, and more particularly, to a method for producing a laminated diffraction optical element for use in an optical apparatus, such as a camera or a video camera, by replica molding using photo-setting resin or thermosetting resin.
Description of the Related Art
Examples of laminated diffraction optical elements molded from photo-setting resin or thermosetting resin are a diffraction optical element, an aspherical lens, and a pickup lens. Replica molding is a typical molding method for these laminated diffraction optical elements. Replica molding includes a step of dropping and supplying resin onto a substrate or a mold having a fine shape, a step of setting the resin by application of energy, and a step of releasing the resin and the substrate together from the mold.
To sufficiently fill the mold with resin in replica molding, for example, a protrusion portion to be filled with squeezing resin is provided on the outer periphery of an optically effective portion of the mold, as disclosed in Japanese Patent Laid-Open No. 5-119202. In the optically effective portion, non-filling is caused, for example, by unevenness in amount of resin dropped on the mold or the substrate and initial misalignment of a lens and the mold. Accordingly, to avoid such non-filling, a generous amount of resin is dropped beforehand to prevent non-filling in the optically effective portion, and a surplus of the resin is supplied in the protrusion portion provided on the outer periphery of the optically effective portion. This allows the optically effective portion to be sufficiently filled with resin so that the resin does not squeeze out of the mold.
However, when resin having low transmittance, for example, resin in which microparticles are dispersed is used in replica molding, a resin layer needs to be uniformly and thinly molded to meet the optical requirement. To be thinly and uniformly molded, the resin needs to be supplied while pressurizing a substrate. When the resin is supplied under pressure, since there is a limit on adjustment of parallelism of axes of the pressed surface, the substrate surface, and the mold surface, the resin is biased according to tilts of the axes. This deteriorates fillability of the resin.
When the technique described in Japanese Patent Laid-Open No. 5-119202 is used to avoid such deterioration of fillability of the resin, the optically effective portion can be sufficiently filled with the resin, but the resin protrusion portion in an optically non-effective outer portion adjacent to the optically effective portion is not uniformly filled with the resin. Further, the roundness of spread resin is seriously decreased by pressurization. As a result, parts filled with resin and parts that are not filled with resin are randomly arranged in the circumferential direction in the protrusion portion of the optically non-effective outer portion. When such a laminated diffraction optical element is mounted in an optical apparatus such as a camera or a video camera, a random filling state in the protrusion portion causes a problem of appearance. Particularly when resin having low transmittance is used, the portions filled with the resin and the portions that are not filled with the resin are easily distinguished because of a great difference in transmittance therebetween. This causes a serious problem from viewpoint of appearance of a product.